Superionic iron oxide–hydroxide in Earth’s deep mantle

Abstract

H2O ice becomes a superionic phase under the high pressure and temperature conditions of deep planetary interiors of ice planets such as Neptune and Uranus, which affects interior structures and generates magnetic fields. The solid Earth, however, contains only hydrous minerals with negligible amount of ice. Here we combine high pressure and temperature electrical conductivity experiments, Raman spectroscopy, and first-principles simulations, to investigate the state of hydrogen in the pyrite type FeO2Hx (x ≤ 1) which is a potential H-bearing phase near the coremantle boundary. We find that when the pressure increases beyond 73 GPa at room temperature, symmetric hydroxyl bonds are softened and the H + (or proton) become diffusive within the vicinity of its crystallographic site. Increasing temperature under pressure, the diffusivity of hydrogen is extended beyond individual unit cell to cover the entire solid, and the electrical conductivity soars, indicating a transition to the superionic state which is characterized by freely-moving proton and solid FeO2 lattice. The highly diffusive hydrogen provides fresh transport mechanisms for charge and mass, which dictate the geophysical behaviors of electrical conductivity and magnetism, as well as geochemical processes of redox, hydrogen circulation, and hydrogen isotopic mixing in Earth's deep mantle.Hydrogen plays an important role in the deep interior of the Earth 1,2 , where its mobility and bonding properties are altered dramatically from localized to globally itinerant with increasing depth. At shallower depths, hydrogen bonds with oxygen, the most abundant element in Earth, to form hydroxyls which modulate the electrical 3,4 , thermal 5 , and elastic 6 properties of the host minerals, and dictate redox, melting, and isotope partitioning 7 . Properties of hydroxyl groups have been extensively studied during the past half century as a means to locate deep water reservoirs and to monitor water circulation for a broad range of applications in interpretation of large geophysical and geochemical features in depth [8][9][10] . Hydroxyl starts with an asymmetric configuration O-H⋯O in which the hydrogen atom between

Iron Hydride in the Earth's Inner Core

“…At the temperature of 360 GPa and 5000 K, we observe that while Fe-H is still solid (as observed in its RDF and RMSD plots, see Figure S7) the H atom shows a continual drift in time which is not seen at 3500 K. This is also observed in a manual inspection of the results as H atoms move between different Fe sites over time. This suggests that above ~3500 K the Fe remains in a solid sublattice but that the H sublattice converts to a reported superionic state 42 where the H atoms can freely move. By determining the slope of the RMSD we determined the H diffusion coe cient to be 10 -8 -10 -9 m 2 /s above the temperature of ~4000 K, which demonstrates that the H has signi cant mobility in Fe-H system.…”

Mathematical modelling of proton migration in Earth mantle

“…Recent observations of prominent electrical conductivity due to superionic proton conduction in the host hydrous mineral (iron oxide-hydroxide ) [14,16] in the deep Earth mantle have brought to concrete realization a paradigm and prediction by Vernadski 100 years ago [19,33]. It is widely accepted that water in the mantle is stored in hydrous minerals.…”

“…Here, the H atom constitutes the unit of mass and energy transport, and the migration of electrons induced by hydrogen is more profound. High-electrical conductivity offers a new perspective for using electromagnetic data to explore the Earth's interior [16].…”

“…The development of a mathematical model for proton migration in the superionic phases [10,14,16] is highly challenging. Through literature survey including experimental results [16], we concluded that mathematical modelling in superionicity research must endeavour to capture the collective motion of charged particles (protons). The primary variable of interest is the self-consistent electromagnetic field potential created by the charges themselves.…”

The Deep Earth Engine Driving Major Surface Events

“…Under the extreme pressures of the deep lower mantle, the hydrogen atoms are no longer tied up in the O-H bond but become H + ion in the structure. At the high temperature of the deep lower mantle, the H + ions are further liberated to become superionic and move freely in and out of the crystal lattice, impacting the electrical, magnetic, thermal and hydrogen fugacity balance of the deep lower mantle (Hou et al, 2021). Recognition of novel materials in deep Earth relies on seismological observations of their characteristic elastic signatures which must be determined in-situ at high pressure-temperature conditions of the deep interior.…”